allow set vint severity

currently, ale has an option `g:ale_vim_vint_show_style_issues` to toggle vint severity setting between `warnings` and `style problems`:

```
let l:warning_flag = ale#Var(a:buffer, 'vim_vint_show_style_issues') ? '-s' : '-w'
```

however I would like to set the severity to errors only, it would be nice if ale exposes an option to set vint severity directly.

btw, because ale has config severity flag in the command line, vint config file has no effect here.